I simply purchased the whole house filter that I preferred (at Lowe's in my case), installed hose fittings and then cut a couple notches in a plastic milk crate. Connect a short leader hose to the supply, then to the filter which sits in the milk crate, then use your normal pressure regulator/potable water set up to connect to the camper. You can change your filters outside so no risk of leaks inside. Done.
2014 Coachmen Catalina 253RKS
2016 Summerland Mini 1750 RD
2012 F250 Lariat FX4 Crew Cab 6.2L